This invention relates to a method of operating a non-linear Mach-Zehnder (MZ) optical modulator.
For high bit rate, long-haul communications supported by optical fibers, high quality light sources serving as transmitters are essential. A typical light source is made up of a laser operating in continuous wave (CW) mode to provide a beam of light to an electro-optical modulator that is controlled to switch the light on and off at the desired bit rate. The modulator may be controlled by the switching of differential control voltages to modify the characteristics of the output light beam according to a modulation format/scheme.
Such formats include simple binary modulation and the more complex duobinary format. Binary modulating a light source is basically a matter of switching the light on to represent a first state and switching the light off to represent a second state. Thus, in a binary modulation scheme there are only two possible states, the xe2x80x98onxe2x80x99 state and the xe2x80x98offxe2x80x99 state, each corresponding to either logic one or logic zero respectively. Alternatively, the duobinary format provides an optical signal more complex than one that can simply be in an xe2x80x98onxe2x80x99 or xe2x80x98offxe2x80x99 state. In fact, there are two xe2x80x98onxe2x80x99 states and one xe2x80x98offxe2x80x99 state. In both of the two xe2x80x98onxe2x80x99 states the light is on with equal intensity, but each xe2x80x98onxe2x80x99 state is xc2x1xcfx80 radians out of phase with the other.
A common modulator structure that is used to realize a number of modulation schemes/formats is the Mach-Zehnder (MZ) interferometer. A MZ interferometer is made up of a pair of waveguide arms connected between an optical waveguide splitter and a waveguide combiner. In operation, a light source (e.g. a laser) is optically coupled to the waveguide splitter that separates the laser""s output light beam into two beams. The two newly separated light beams travel from the splitter into and through the pair of waveguide arms and are recombined within the waveguide combiner. Electrodes are associated with each of the waveguide arms and by applying a modulating voltage to one or both electrodes the relative phases of the two light beams can be altered.
There are also many design options that may be exercised that have the effect-of establishing a built-in phase shift between two beams of light within an interferometer. For example, a built-in phase shift can be established by a careful selection of the splitter (or coupler) design, choosing different lengths for each of the interferometer arms, or by adding separate electrodes that can be used to apply various drive voltages to induce different desired phase shifts. Specifically, as described in Yu (U.S. Pat. No. 5,991,471), the entire contents of which are herein incorporated by reference, the interferometer arms can be configured to induce a built-in phase shift between the two beams of light in the absence of a differential voltage applied between the two waveguide arms.
Currently, however, the only practical way in which to realize a duobinary signalling device (modulator) is to employ a Mach-Zehnder (MZ) interferometer based on a linear and real refractive index change mechanism known as a linear MZ optical modulator. However, linear MZ optical modulators have a number of drawbacks that limit their utility in some communication spaces. The drawbacks of linear MZ optical modulators include their relatively large size, high production cost and the high power consumption of their electrical drivers.
The aforementioned drawbacks stem from the fact that this type of modulator is commonly fabricated in a lithium niobate (LiNbO3) process. Lithium niobate modulators are typically on the order of at least three inches long, and there are no foreseeable fabrication technologies in the art that would enable the construction of truly linear MZ optical modulators that could be on the order of a few millimeters in length.
MZ interferometers (modulators) can be fabricated from III-V semiconductor alloy materials such as Indium Phosphide (InP) and Indium Gallium Arsenic Phosphide (InGaAsP), employing structures having multiple quantum wells (MQW) and barriers, that enable the manufacture of MZ interferometers that are only a few millimeters in length.
All MZ modulators (interferometers) convert phase modulation into intensity modulation and it is important that the amplitude ratio between an xe2x80x98onxe2x80x99 state and the xe2x80x98offxe2x80x99 state is relatively high. This ratio, known as the extinction ratio (ER), is a measure of the signal intensity against the background noise. Beneficially, a high ER also permits greater span between repeaters in an optical transmission network.
Accordingly, thus far, as described in Yu (U.S. Pat. No. 5,991,471) and Rolland et al. (U.S. Pat. No. 5,524,076), the entire contents of both herein incorporated by reference, MQW III-V MZ optical modulators have been primarily used to produce simple binary modulators. Specifically, Rolland et al. discloses a simple binary modulation method, in which the output of the modulator may be considered either xe2x80x98onxe2x80x99 or xe2x80x98offxe2x80x99 based on its relative intensity without regard to phase of the output beam in the xe2x80x98onxe2x80x99 state (Col. 2, lines 1-18).
According to a first aspect, the invention provides a method of operating a non-linear Mach-Zehnder (MZ) optical modulator. The non-linear MZ optical modulator has first and second waveguide arms and corresponding first and second electrodes connected to the respective first and second waveguide arms. The method of operating the non-linear MZ optical modulator comprises a) operating the non-linear MZ optical modulator in a duobinary modulation format by: i) biasing both waveguide arms via the corresponding first and second electrodes at a first value of Vxcfx80/2 volts to obtain a duobinary xe2x80x98offxe2x80x99 output signal characteristic of a duobinary xe2x80x98offxe2x80x99 state; ii) biasing the first waveguide arm via the first electrode at zero volts and biasing the second waveguide arm via the second electrode at Vxcfx80 volts to obtain a first duobinary xe2x80x98onxe2x80x99 output signal characteristic of a first duobinary xe2x80x98onxe2x80x99 state; and, iii) biasing the second waveguide arm via the second electrode at zero volts and biasing the first waveguide arm via the first electrode at Vxcfx80 volts to obtain a second duobinary xe2x80x98onxe2x80x99 output signal characteristic of a second duobinary xe2x80x98onxe2x80x99 state.
In some embodiments of the invention the duobinary modulation defines: i) the first duobinary xe2x80x98onxe2x80x99 state as having the first duobinary xe2x80x98onxe2x80x99 output signal characterized as when the optical output power of the non-linear MZ optical modulator is at its peak intensity with a zero radian phase shift; ii) the second duobinary xe2x80x98onxe2x80x99 state as having the second duobinary xe2x80x98onxe2x80x99 output signal characterized as when the optical output power of the non-linear MZ optical modulator is at its peak intensity with a xc2x1xcfx80 radian phase shift; and iii) the duobinary xe2x80x98offxe2x80x99 state as having the duobinary xe2x80x98offxe2x80x99 output signal characterized as when the optical output power of the non-linear MZ optical modulator is substantially non-existent. Moreover, in some embodiments that the first and second duobinary xe2x80x98onxe2x80x99 output signals and the duobinary xe2x80x98offxe2x80x99 output signal are substantially free from frequency chirp.
Also, in some embodiments, the non-linear MZ optical modulator is a Multiple Quantum Well (MWQ) III-V MZ optical modulator; And, additionally, the non-linear MWQ III-V MZ optical modulator is adapted to include a xc2x1xcfx80 radian phase shift between waveguide arms comprising the MWQ III-V MZ optical modulator. In such embodiments it would be preferable that the III-V material comprising the Multiple Quantum Well (MQW) III-V MZ optical modulator is Indium Phosphide (InP) and the multiple quantum wells are composed of a Indium Gallium Arsenide Phosphide (InGaAsP) alloy. However, the non-linear MZ optical modulator may alternatively be comprised of III-V bulk materials or polymer materials.
In some embodiments method the value Vxcfx80/2 is the minimum voltage at which the phase of the non-linear MZ optical modulator output is xc2x1xcfx80/2 radians when a drive voltage is applied to one waveguide arm via the corresponding electrode, as for a one arm modulation procedure. Similarly, the value Vxcfx80 is the minimum voltage at which the phase of the non-linear MZ optical modulator output is xc2x1xcfx80 radians when a drive voltage is applied to one waveguide arm via the corresponding electrode, as for a one arm modulation procedure.
According to yet another aspect of the invention a duobinary modulator for delivering an optical signal according to a duobinary modulation format is provided. The duobinary modulator comprises: a) a non-linear Mach-Zehnder (MZ) optical modulator having first and second waveguide arms and corresponding first and second electrodes connected to the respective first and second waveguide arms; and b) an asymmetric drive circuit to provide a set of three asymmetric drive conditions. The set of three asymmetric drive conditions provides: i) biasing to both waveguide arms via the corresponding first and second electrodes at a first value of Vxcfx80/2 volts to obtain a duobinary xe2x80x98offxe2x80x99 output signal characteristic of a duobinary xe2x80x98offxe2x80x99 state; ii) biasing to the first waveguide arm via the first electrode at zero volts and biasing to the second waveguide arm via the second electrode at Vxcfx80 volts to obtain a first duobinary xe2x80x98onxe2x80x99 output signal characteristic of a first duobinary xe2x80x98onxe2x80x99 state; and, iii) biasing to the second waveguide arm via the second electrode at zero volts and biasing to the first waveguide arm via the first electrode at Vxcfx80 volts to obtain a second duobinary xe2x80x98onxe2x80x99 output signal characteristic of a second duobinary xe2x80x98onxe2x80x99 state. Again it is preferred that the first and second duobinary xe2x80x98onxe2x80x99 output signals and the duobinary xe2x80x98offxe2x80x99 output signal are substantially free from frequency chirp.
A third aspect of the invention provides a method of operating a non-linear Mach-Zehnder (MZ) optical modulator having first and second waveguide arms and corresponding first and second electrodes connected to the respective first and second waveguide arms, the method comprising: a) operating the non-linear Mach-Zehnder (MZ) optical modulator in a simple binary format by: i) biasing both waveguide arms via the corresponding first and second electrodes at a first value of Vxcfx80/2 volts to obtain an xe2x80x98offxe2x80x99 output signal; ii) biasing the first waveguide arm via the first electrode at zero volts and biasing the second waveguide arm via the second electrode at Vxcfx80 volts to obtain an xe2x80x98onxe2x80x99 output signal. In some embodiments, both the xe2x80x98onxe2x80x99 output signal and the xe2x80x98offxe2x80x99 output signal are substantially free from frequency chirp.
Another aspect of the invention provides a method of operating a non-linear Mach-Zehnder (MZ) optical modulator having first and second waveguide arms and corresponding first and second electrodes connected to the respective first and second waveguide arms, the method comprising: i) biasing both waveguide arms via the corresponding first and second electrodes at a first value to obtain an xe2x80x98offxe2x80x99 output signal; ii) biasing the first waveguide arm via the first electrode at a second value and biasing the second waveguide arm via the second electrode asymmetrically in relation to the first waveguide arm from the first value at a third value to obtain an xe2x80x98onxe2x80x99 output signal. In some embodiments, the asymmetric biasing provided substantially compensates for the non-linearity of the non-linear MZ optical modulator such that both the xe2x80x98onxe2x80x99 output signal and the xe2x80x98offxe2x80x99 output signal are substantially free from frequency chirp.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.